Semiconductor light emitting apparatus and method of fabricating the same

ABSTRACT

A semiconductor light emitting apparatus is proposed, which has thyristor without increasing number of constituent semiconductor layers, with large degree of freedom of selection of ON voltage. It comprises first-conductivity-type first cladding layer, active layer, and second-conductivity-type second cladding layer on substrate; pair of opposing first recesses forming stripe-patterned ridge configuring major current path, and second recess disposed on outer side of one of first recesses on second cladding layer side, each recesses formed to depth keeping active layer unreached; first-conductivity-type current blocking layer formed over inner surfaces of first and second recesses, and second-conductivity-type contact layer formed on current blocking layer; wherein light emitting portion and thyristor structural portion composed of stack of second-conductivity-type contact layer, first-conductivity-type current blocking layer, second-conductivity-type cladding layer, active layer and first-conductivity-type first cladding layer at second recess are formed; and ON voltage of thyristor is adjustable on selection of depth of second recess.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Priority Application No.2003-366336, filed on Oct. 27, 2003 with the Japanese Patent Office, theentire content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor light emittingapparatus having a semiconductor light emitting element such assemiconductor laser, light emitting diode or the like, and a method offabricating the same, aiming to prevent destruction, such aselectrostatic destruction, of the semiconductor light emitting element,and to make a great stride of improvement in the strength againstelectrostatic destruction.

2. Description of Related Art

Electrostatic destruction, or electrostatic allowance, is an essentialissue of semiconductor light emitting apparatus typically havingsemiconductor laser element. The electrostatic destruction iscategorized into that ascribable to short-circuiting of thesemiconductor laser per se, and that ascribable to destruction of lightemission end face due to instantaneous increase in the emissionintensity. Generally speaking, efforts of improving performances of thelaser, such as for lower power consumption and higher efficiency, tendto degrade the electrostatic allowance. This is supposed that decreasein the threshold carrier density J_(th) and improvement in thedifferential quantum efficiency possibly worsen damage on thesemiconductor laser even under an equivalent level of staticelectricity. The above-described end face destruction can be solved byadopting so-called window structure which can moderate concentration oflight in the end face structure, wherein general methods adopted for thepurpose of avoiding the electrostatic destruction including this sort ofdestruction and of raising the electrostatic allowance, include use ofprotection elements such as capacitor, diode, thyristor and so forth,which are externally attached. This, however, results in increase in thecost due to increase in the number of process steps or components forthe assembly, and increase in the size.

On the other hand, a proposal has been made on a configuration in whicha thyristor composing a protection circuit is fabricated into asubstrate composing a semiconductor light emitting element (see, forexample, Japanese Patent Application Publication No. HEI5-67849, FIG.1). However, thus-configured semiconductor light emitting apparatussuffers from a complicated configuration such as having a large numberof stacked semiconductor layers, and has only a small degree of freedomon selection of characteristics of the thyristor, because voltage andother characteristics of the thyristor can affect characteristics of thesemiconductor light emitting apparatus.

However, for an exemplary case where an active layer of a semiconductorlight emitting adopts a multiple quantum well structure, characteristicsof the apparatus will vary, which is typified by increase in the ONvoltage as compared with that in a single-layer structure, so that itwill be necessary to correspondently alter a design value of the ONvoltage V_(on) of the thyristor.

SUMMARY OF THE INVENTION

The present invention is to provide a semiconductor light emittingapparatus and a method of fabricating the same, which make it possibleto fabricate a thyristor, which is provided for preventing so-calledelectrostatic destruction, or destruction induced by an accidentalover-current, together with a semiconductor light emitting apparatus,without increasing the number of constituent semiconductor layers of adesired semiconductor light emitting element, and without relying upon aspecial fabrication technique, and moreover, also make it possible toselect characteristics such as ON voltage of the thyristor with a largedegree of freedom, while giving almost no influence on setting ofcharacteristics of the semiconductor light emitting apparatus.

A semiconductor light emitting apparatus of the present invention isconfigured as having, on a substrate, at least a first-conductivity-typefirst cladding layer, an active layer, and a second-conductivity-typesecond cladding layer; having, on the second cladding layer side, a pairof opposing first recesses forming a stripe-patterned ridge whichconfigures a major current path, and a second recess disposed on theouter side of at least one of the first recesses, each of the first andsecond recesses being formed to a depth while keeping the active layerunreached; having a first-conductivity-type current blocking layerformed so as to extend over the inner surfaces of the first and secondrecesses, and a second-conductivity-type contact layer formed on thecurrent blocking layer; wherein a light emitting portion causing lightemission by current which flows through the major current path; and athyristor structural portion composed of a stack of thesecond-conductivity-type contact layer, the first-conductivity-typecurrent blocking layer, the second-conductivity-type cladding layer, theactive layer and the first-conductivity-type first cladding layer at thesecond recess are formed; and having ON voltage of the thyristorstructural portion adjustable depending on selection of depth of thesecond recess.

In the semiconductor light emitting apparatus of the present invention,the active layer can be configured as having a multiple quantum well(MQW) structure. In the semiconductor light emitting apparatus of thepresent invention, it is also allowable to form a stripe-patternedboundary ridge between the first recess and the second recess, so as toextend along the stripe-patterned ridge. The ON voltage of the thyristoris specifically adjusted to a low value not affective to operations ofthe semiconductor light emitting element. Operational voltage of thethyristor after turned on is adjusted to lower than, or equivalent tothe operation voltage of the semiconductor light emitting element. Areaof the thyristor structural portion is adjusted larger enough than areaof the major current path portion.

A method of fabricating a semiconductor light emitting apparatus of thepresent invention is characterized by having a step of forming, on asubstrate by epitaxial growth, at least a first-conductivity-type firstcladding layer, an active layer and a second-conductivity-type secondcladding layer, composing a semiconductor light emitting element; a stepof forming, on the second cladding layer side, a pair of opposing firstrecesses forming therebetween a stripe-patterned ridge which configuresa major current path; a step of forming a second recess on the outerside of at least one of the first recesses; a step of epitaxiallygrowing a first-conductivity-type current blocking layer and asecond-conductivity-type contact layer so as to extend over the firstand second recesses; a step of forming a recess to a depth across thecontact layer and current blocking layer above the ridge to thereby forma thinned portion; and a step of introducing a second-conductivity-typeimpurity from the side above the contact layer to thereby form, in thethinned portion, a contact portion reaching the second cladding layer ofthe ridge, wherein a light emitting portion causing light emission bycurrent which flows through the major current path; and a thyristorstructural portion composed of a stack of the second-conductivity-typecontact layer, the first-conductivity-type current blocking layer, thesecond-conductivity-type cladding layer, the active layer and thefirst-conductivity-type first cladding layer at the second recess areformed; and having ON voltage of the thyristor structural portionadjustable depending on selection of depth of the second recess.

According to the above-described semiconductor light emitting apparatusof the present invention, the thyristor based on an alternative stack ofthe first and second conductivity-type layers is configured in thesecond recess, using only semiconductor layers composing thesemiconductor light emitting element, by forming the second recess onthe outer side of the first recesses composing the ridge therebetween,by filling the blocking layer for limitedly forming the current path inthe ridge so as to extend over the second recess, and further by formingthereon the contact layer, and thereby the thyristor can be configuredas having a PNPQN structure referring now the active layer, adopting amultiple quantum well structure, to as Q. Because the thyristor andsemiconductor light emitting element are in an electrically parallelconfiguration, an appropriate selection of the ON voltage of thethyristor makes it possible to effectively prevent destruction of thesemiconductor light emitting element, even if an abnormal voltage isaccidentally applied to the semiconductor light emitting element, bypreliminarily turning the thyristor on.

The ON voltage V_(on) of the thyristor in the configuration of thepresent invention is adjustable by the depth of the second recess, anddestruction voltage varies depending on the configuration in particularfor the case where the active layer adopts a multiple quantum wellstructure, and this raises a need of an adaptive design of the ONvoltage of the thyristor. The configuration of the present inventionmakes it possible to select the ON voltage through selection of thedepth of the second recess, without affecting characteristics of thesemiconductor light emitting element per se, while successfully avoidingany need of addition of a specialized semiconductor layer forcontrolling the ON voltage.

The structure additionally having the boundary ridge between the firstand second recesses, that is, the second recess having a ridge-forminggroove structure, is more successful in preventing the characteristicsof the light emitting element portion from being affected by thethyristor portion, and more specifically, by the thyristor portionduring the ON time

Selection of the area of the thyristor structural portion as largerenough as possible than the area of the major current path portion ofthe ridge portion makes it possible to allow a larger amount of currentto flow through the thyristor when it is turned on, and this furtherimproves electrostatic destruction strength of the semiconductor lightemitting element, which is typically semiconductor laser.

Further features of the invention, and the advantages offered thereby,are explained in detail hereinafter, in reference to specificembodiments of the invention illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of one example of a semiconductorlight emitting apparatus of the present invention;

FIG. 2 is a current-voltage characteristic curve of the semiconductorlight emitting element and the thyristor structural portion forexplaining characteristics of the semiconductor light emitting apparatusof the present invention;

FIG. 3 is a schematic sectional view of an essential portion of thesemiconductor light emitting apparatus in one process step of anexemplary method of fabricating the same according to the presentinvention;

FIG. 4 is a schematic sectional view of an essential portion of thesemiconductor light emitting apparatus in one process step of anexemplary method of fabricating the same according to the presentinvention;

FIG. 5 is a schematic sectional view of an essential portion of thesemiconductor light emitting apparatus in one process step of anexemplary method of fabricating the same according to the presentinvention; and

FIG. 6 is a schematic sectional view of an essential portion of thesemiconductor light emitting apparatus in one process step of anexemplary method of fabricating the same according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The next paragraphs will describe a mode of embodiment of thesemiconductor light emitting apparatus according to the presentinvention, referring to a schematic sectional view shown in FIG. 1. Itis to be noted that the present invention is by no means limited to thismode of embodiment. The case exemplified herein relates to anAlGaAs-base semiconductor laser apparatus in a 780-nm band, and has, ona base 1 which is a first-conductivity-type or typically n-type GaAssubstrate, an n-type first-conductivity-type or n-type buffer layer,which is expressed in the drawing as of double-layered typicallycomprising a GaAs second buffer layer 2 and an AlGaAs second bufferlayer 3; a first-conductivity-type or n-type AlGaAs first cladding layer4; an active layer 5 having a multiple quantum well (MQW) structure inwhich GaAs and AlGaAs are alternatively stacked; and a second claddinglayer 6 in which a lower second cladding layer 6L and ahigh-concentration upper second cladding layer 6H, both of which being asecond-conductivity-type or p-type, are stacked.

On the second cladding layer 6 side, there are provided a pair ofridge-forming grooves, for example, for configuring a pair of opposingfirst recesses 8 for forming a stripe-patterned ridge 9 which configuresthe main current path; and ridge-forming grooves, for example, forconfiguring the second recesses 10 formed on the outer side of at leastone of the first recesses 8, and typically on the outer sides of both ofthem. Each of the first and second recesses 8, 10 is formed to a depthwhile keeping the active layer 5 unreached, wherein the recess 10 istypically formed to a depth so that it runs across thehigh-concentration upper second cladding layer 6H. By the proceduresdescribed in the above, stripe-patterned boundary ridges 20 are formedbetween the first recesses 8 and second recesses 10, so as to extendalong the stripe-patterned ridge 9.

A first-conductivity-type or n-type current blocking layer 11 is thenformed so as to extend over the inner surfaces of the first and secondrecesses 8, 10, and a second-conductivity-type or p-type contact layer12 is formed further on the current blocking layer 11. Thisconfiguration makes it possible to form, in the active layer 5, thelight emitting portion with the aid of current which flows through themajor current path, and at the same time makes it possible to form PNPQNswitching elements or thyristor structural portions 21 at the secondrecesses 10 by a stack arranged thereon and thereunder, wherein thestack comprises the second-conductivity-type or p-type contact layer 12;the first-conductivity-type or n-type current blocking layer 11; thesecond-conductivity-type or p-type cladding layers 6L and 6H; the activelayer (Q) having a multiple quantum well structure; and the firstcladding layer 4, the buffer layers 3 and 2, and the base 1,respectively having the first conductivity type or n-type conductivity.The ON voltage Von of the thyristor structural portion 21 is selectablethrough selection of the depth of the second recess 10, depending onimpurity concentration and thickness of the current blocking layer 11,and on impurity concentration of the contact layer 12.

In thus-configured semiconductor light emitting apparatus of the presentinvention, the individual current-voltage characteristics of theintrinsic semiconductor light emitting element 22 having the majorcurrent path formed in the ridge 9 and the PNPQN structural portions areselected, as being respectively indicated by a solid line and a brokenline in FIG. 2, so that the ON voltage V_(on) of the thyristors is setclose to the threshold voltage V_(th) or operation voltage V_(op) of thesemiconductor light emitting element, so far as it does not interferethe operations of the semiconductor light emitting element. For anexemplary case where the operation voltage V_(op) of the semiconductorlight emitting element is 3 V, setting of V_(on) to 4 V to 10 V oraround, and setting of the operation voltage of the turned-on thyristorlower than or equivalent to the operation voltage V_(op) of thesemiconductor light emitting element make it possible to certainlyprevent electrostatic destruction of the semiconductor light emittingelement from occurring. In addition, area of the above-describedthyristor structural portions is selected sufficiently larger than thatof the major current path portion in the ridge 9.

Next paragraphs will explain one exemplary mode of embodiment of themethod of fabricating a semiconductor light emitting apparatus accordingto the present invention, referring to process charts in FIG. 3 to FIG.6. This example relates to a case of fabrication of the light-emittingsemiconductor apparatus explained referring to FIG. 1, wherein first asshown in FIG. 3, the base 1, which is typically afirst-conductivity-type or an n-type GaAs substrate, is obtained, and afirst epitaxial growth process is carried out on the base 1, in whichthe first buffer layer 2 of 5000 Å thick for example composed offirst-conductivity-type or n-type GaAs, the second buffer layer 3 of5000 Å thick for example similarly composed of first-conductivity-typeor n-type GaAs, the first cladding layer 4 of 2 μm thick for examplecomposed of first-conductivity-type or n-type Al_(x)Ga_(1−x)As, theactive layer 5, the second cladding layer 6L of 4000 Å thick for examplecomposed of second-conductivity-type or p-type Al_(x)Ga_(1−x)As, and thehigh-impurity-concentration second cladding layer 6H of 1 μm thick forexample composed of second-conductivity-type or p-type Al_(x)Ga_(1−x)Asare sequentially stacked by epitaxial growth typically through the MOCVD(Metal Organic Chemical Vapor Deposition) process.

The active layer 5 can be configured, as described in the above, by themultiple quantum well structure (referred to as MQW, hereinafter)typically based on alternative stacking of AlGa layers and AlGaAslayers. In so-called SCH (Separate Confinement Heterostructure), guidelayers are epitaxially grown on the upper side and lower side of theactive layer 5 while placing it in between.

Next, as shown in FIG. 4, a pair of first ridge-forming grooves 8 areformed by patterning-by-etching typically based on RIE (Reactive IonEtching) through a mask formed by photolithography, to a depth acrossthe high-concentration upper second cladding layer 6H and to as deep asthe lower second cladding layer 6L having a lower impurity concentrationthan the clad layer 6H has, to thereby form the ridge 9 between theridge-forming grooves 8. On the outer side of the ridge-forming grooves8, a pair of second ridge-forming grooves 10 are formed along the firstridge-forming grooves 8, similarly by RIE for example. The depth of thesecond ridge-forming grooves 10 is configured that the lower secondcladding layer 6L is left by etching to the thickness of, for example,20 nm. The width of the ridge-forming grooves 8 is preferably more than10 μm, for example, 20 μm. It is necessary to consider the lightdistribution in the active layer 5 when there is a difference in therefractive index of the clad layers 6L, 6H, the current blocking layer11, and the contact layer 12. And the width of the second ridge-forminggrooves 10 is, for example, 100 μm so that it is enable to make the mostof the lateral dimension of the semiconductor light emitting element.The distance between the first ridge-forming groove 8 and the secondridge-forming groove 10 is, for example, 20 μm.

As shown in FIG. 5, a second epitaxial growth process is carried out onthe high-concentration second cladding layer 6H, in which the currentblocking layer 11 of 6000 Å thick for example composed offirst-conductivity-type or n-type AlGaAs of impurity concentrationranging from 10¹⁸ to 10¹⁹ cm³¹ ³, and the contact layer 12 of 9000 Åthick for example composed of high-impurity-concentration or p-type GaAsof high impurity concentration ranging from 10¹⁹ to 10²⁰ cm⁻³ aresequentially grown through epitaxial growth typically by the MOCVDprocess, so as to extend over the inner surfaces of the ridge-forminggrooves 8 and 10.

Next, as shown in FIG. 6, a portion of the stack of the contact layer 12and current blocking layer 11 on the ridge 9 is etched to apredetermined depth typically by patterning-by-etching based on RIE andphotolithography, to thereby form the thinned portion 13. Thereafter, asshown in FIG. 1, a second-conductivity-type or p-type impurity, which istypically Zn, is allowed to diffuse over the entire surface from theside above the contact layer 12 to as thick as 5000 Å for example, tothereby make the thinned portion 13 have a high-concentration secondconductivity type or p-type, and to thereby form a contact layer 14.Next over the entire surface of the contact layers 14 and 12, a firstelectrode 15 composed of a Ti/Pt/Au stack is formed by deposition so asto form an ohmic contact, and on the back surface of the base 1, asecond electrode 16 composed of an Au/Ni/Au stack is deposited so as toform an ohmic contact.

By the procedures described in the above, a desired semiconductor laser,that is, a semiconductor light emitting element, is formed in the ridge9, and on both sides of the second recesses 10, the thyristor structuralportions 21 are formed.

In the above-described fabrication method of the present invention, therecesses 10 are formed to a depth which should be determined by thethyristor structural portion 21, or in other words, to a depthpreliminarily designed so as to select the ON voltage V_(on) of thethyristor, wherein the depth of the second recess 10 can be regulated bya method preliminarily forming a p-type etching stop layer, for example,showing a sufficiently small etchrate in the etching for forming therecesses 10, at a predetermined position of the recesses 10 in theabove-described first epitaxial growth step, and typically in theepitaxial growth process of the cladding layer 6, to thereby facilitatecontrol for termination of the etching. It is also allowable to adopt amethod of controlling the etching time depending on the impurityconcentration of the cladding layer 6, MQW structure of the active layer5, and the impurity concentration of the current blocking layer 11.

It is to be noted that the semiconductor light emitting apparatus of thepresent invention is by no means limited to the case having only asingle semiconductor light emitting element, but includes semiconductorlight emitting elements of various configurations, such as those havinga plurality of semiconductor light emitting elements, or such as thosehaving a semiconductor light emitting element together with any othercircuit elements to thereby configure an integrated circuit. Needless tosay, it is also possible to obtain a large number of semiconductor lightemitting apparatuses at the same time, by forming a large number of thesemiconductor light emitting apparatuses on the base 1 at the same time,and by pelletizing.

The description in the above exemplified a semiconductor light emittingapparatus typically having an AlGaAs-base semiconductor laser of a780-nm wavelength band used for CD (Compact Disc), but the presentinvention is also applicable, for example, to semiconductor lightemitting apparatuses typically using semiconductor light emittingelements of 650-nm wavelength band used as a light source for DVD(Digital Versatile Disc), such as AlGaInP-base semiconductor laser. Thepresent invention is by no means limited to the above-describedexamples, and allows various modifications and alterations withoutdeparting from the spirit thereof, and is applicable to a configurationin which light emitting elements having two or more differentwavelengths, which are typically of 780-nm band and of 650-nm band, areformed as being arranged on a common base. The scope of the invention isindicated by the appended claims, rather than the foregoing description,and all changes that come within the meaning and range of equivalencethereof are intended to be embraced therein.

1-6. (canceled)
 7. A method of fabricating a semiconductor lightemitting apparatus comprising the steps of: eptitaxially growing atleast a first-conductivity-type first cladding layer, an active layerand a second-conductivity-type second cladding layer, composing asemiconductor light emitting element; forming a pair of opposing firstrecesses which extends into the second cladding layer thereby formingtherebetween a stripe-patterned ridge which provides a major currentpath; forming at least one second recess on the outer side of at leastone of said first recess; epitaxially growing a first-conductivity-typecurrent blocking layer and a second and a second-conductivity-typecontact layer so as to extend over said first and second recesses;forming a recess to a depth across said contact layer and currentblocking layer above the ridge to thereby form a thinned portion; andintroducing a second-conductivity-type impurity from the side above saidcontact layer to thereby form, in said thinned portion, a contactportion reaching said second cladding layer of the ridge, wherein alight emitting portion causing light emission by current which flowsthrough said major current path; and a thyristor structural portioncomposed of a stack of said second-conductivity-type contact layer, saidfirst-conductivity-type current blocking layer, saidsecond-conductivity-type cladding layer, said active layer and saidfirst-conductivity-type first cladding layer at said second recess areformed.
 8. A method of fabricating a semiconductor light emittingapparatus according to claim 7, wherein said active layer can beconfigured as having a multiple quantum well (MQW) structure.
 9. Amethod of fabricating a semiconductor light emitting apparatus accordingto claim 7, wherein a stripe-patterned boundary ridge is formed betweensaid first recess and said second recess, so as to extend along saidstripe-patterned ridge.
 10. The method of fabricating a semiconductorlight emitting apparatus according to claim 7, wherein the firstconductivity type is P-type.
 11. The method of fabricating asemiconductor light emitting apparatus according to claim 7, wherein thefirst conductivity type is N-type.